On Debian, the installer will detect systemd and attempt to use it if it is simply present, rather than actually running.
Normally, if you switch from systemd to sysv, you install “sysv-rc”, and the package manager will uninstall “systemd-sysv”. A separate package called “systemd” will remain, even though sysv is being used for init. That “systemd” package can be removed later (after a reboot), but doesn’t have to be.
The error given when this misdetection occurs is rather confusing since everything that fails is based entirely on the idea that systemd was found, while the failures are all purely because systemd is not really being used at all.